Sealing apparatus for sealing sheets of thermoplastic material are well known in the prior art. Typically, the sealing apparatus may include a pair of relatively movable sealing jaws, with one or both of the faces of the opposed sealing jaws being formed of rubber or similar material, or being coated with Teflon or similar material, to prevent sticking of the thermoplastic material to the jaws during the sealing operation. In addition, at least one of the sealing jaws typically includes heating means, such as an impulse wire, which when charged with current, is heated to the desired sealing or welding temperature. The impulse wire thereby operates to fuse adjacent sheets of thermoplastic material to each other.
Such sealing apparatus is typically employed in shrink-wrapping of packages or for merely sealing plastic pouches or packages. In such cases, the thermoplastic sheets are not under tension, and there is not tendency to move the thermoplastic sheets relative to the sealing jaws. In fact, if conventional sealing jaws were employed to heat seal thermoplastic sheets in a stretch-wrapping situation wherein the sheets are under tension, the thermoplastic sheets would tend to slide or slip relative to the Teflon-coated or silicone rubber jaws, as conventional sealing jaws do not provide a sufficient clamping force to overcome the tension on the thermoplastic sheets which is required in stretch-wrapping apparatus.
In addition, in conventional sealing jaws, the heating means, such as the impulse wire, protrudes from the surface of the sealing jaw. Accordingly, if a thermoplastic sheet was to be drawn through the jaws when the jaws are in their open position, the impulse wire would operate to damage or cut the thermoplastic sheet which is passing over it during a stretch-wrapping operation. Accordingly, conventional sealing jaws would not be suitable in stretch-wrapping operations wherein the thermoplastic sheets are drawn between the sealing jaws before the sealing operation takes place.
There are also new materials being employed in stretch-wrapping operations which are not susceptible to heat sealing on conventional sealing jaws. More particularly, foam-type thermoplastic materials are now being employed in stretch-wrapping operations. Such foam-type thermoplastics, such as Dupont's Microfoam, fabricated from a polypropylene resin, cannot be heat sealed on conventional apparatus. Such Microfoam material, upon being heated to its melting point, simply vaporizes and does not allow the sheets to fuse to each other. Accordingly, it would also be desirable to provide sealing jaws while would operate to heat seal such foam-type thermoplastic materials to provide a sufficiently strong seal.
Broadly, it is an object of the present invention to provide improved sealing means which overcome one or more of the aforesaid problems. Specifically, it is within the contemplation of the present invention to provide improved sealing jaws which are constructed so as to be suitable for heat sealing thermoplastic sheets which are under tension, and particularly those which are utilized in stretch-wrapping operations.
It is a further object of the present invention to provide improved sealing jaws which may be utilized to heat seal thermoplastic sheets which are under tension, such as in stretch-wrapping operations, wherein the sealing jaws provide a sufficient clamping force to overcome the tension applied to the thermoplastic sheets and to prevent slippage of the thermoplastic sheets relative to the sealing jaws during the sealing operation.
It is a still further object of the present invention to provide improved sealing jaws which are constructed to prevent the thermoplastic sheets which are drawn between the sealing jaws from engaging the heating means so as to prevent the thermoplastic sheet from being damaged or torn by the heating means.
It is a still further object of the present invention to provide improved sealing jaws which are especially suitable for heat sealing foam-type thermoplastic materials to provide a sufficiently strong seal.